5 Experimental work
5.2 Absorption measurements
5.2.1 Experimental setup and verification
The initial absorption measurements were done with penray lamps (Hg at 185 nm and Zn at 214 nm). The setups for both penray experiment are shown in Figure 5-2. The flow cell was set up the same as it was for kinetics experiments, but the optics for the excimer laser, D2 lamp, and diode laser were all removed to allow positioning of the penray lamp and photomultiplier tube (PMT) detector. A different PMT with a different photocathode was used depending on the penray lamp; a Cs-I photocathode was used for the Hg penray and a bialkali photocathode was used for the Zn penray. To obtain
sufficient signal the Zn setup also required using 6 and 5 cm focal length optics at the entrance and exit of the cell, respectively. A picoammeter was used to read out the signals in both setups.
Figure 5-2. Setup for penray lamp experiments. To look at the 185 nm line from the Hg lamp a Cs-I photocathode PMT was used to reduce all light at λ > 190 nm. For the 214 nm line from the Zn lamp it was necessary to use focusing optics before and after the cell with a bialkali photocathode PMT. For both lamps a filter was used at λ = 185 or 214 nm to attenuate other wavelengths.
Absorption experiments were also done using the D2 lamp as the light source, and were setup exactly as the normal kinetics experiments were in Figure 5-1. In a few cases a Cary UV-Vis spectrometer was used to verify absorption cross sections. A list of all the molecules that were studied including the method and λ that they were studied at is given in Table 5-1.
Table 5-1. Summary of all the molecules and λ where absorption measurements were made.
Molecule λ (nm)
source σfull cell / (cm2)
σlit / (cm2)
Path Length (cm) (100 Torr, 10 s res , 100% purge) CH3OH 185 Hg penray 6.30 x 10-19 b
N2O 214 Zn penray 3.35 x 10-21 3.60 x 10-21 b N2O 214 Cary 3.37 x 10-21 a
N2O 185 Hg penray 1.27 x 10-19 1.40 x 10-19 c N2O 185 Cary 1.36 x 10-19 a
CF3Br 185 Hg penray 4.23 x 10-20 4.45 x 10-20 c CF3Br 185 Cary 4.44 x 10-20 a
CF3Br 220 D2 lamp 7.45 x 10-20 7.56 x 10-20 c 149 Cl2 295 D2 lamp 8.52 x 10-20 8.85 x 10-20 c 149 Acetone 275 D2 lamp 4.96 x 10-20 4.96 x 10-20 b 161
aMeasurements were made in a 10 cm cell in the Cary spectrometer, b MPI-Mainz UV database,3
c JPL-06 recommendation4
Absorption measurements were first attempted to verify the [CH3OH] in the flow cell. The 185 nm line of the Hg penray lamp was used as a light source because of the suitable absorption cross section of CH3OH at this wavelength. Measurements of N2O at 185 and 214 nm, and CF3Br at 185 nm followed in order to check the experimental setup.
CH3OH was introduced into the cell by bubbling N2 through liquid CH3OH held at 0 °C (V.P. 30 Torr)5 and monitoring the total pressure in the bubbler. The N2O and CF3Br were supplied directly from gas cylinders. Nitrogen calibrated flow meters were used to monitor the flows of all gases by using the appropriate correction factors (0.75 for N2O and 0.37 for CF3Br) where necessary. The concentrations of each species in molecules cm-3 were: [CH3OH] = 5 x 1014 – 1 x 1016, [N2O] (185 nm) = 9 x 1015 – 1.8 x 1017, [N2O]
(214 nm) = 3 x 1017 – 2 x 1018, and [CF3Br] = 3 x 1016 – 3 x1017. The high voltage supplied to the PMTs was typically between 600 and 900 V.
Measurements were performed by measuring the intensity of light coming
through the cell without any of the absorbing gas present, and then by measuring the light with a series of 4 – 5 different concentrations of the absorber present. A check of the baseline intensity was performed after evacuating the cell, and the measurements repeated. Fluctuations in the measured signal were a common problem. The estimated precision of the signal measurements from the picoammeter due to these fluctuations was
± 0.005 µA.
The first attempts at penray measurements using the 185 nm Hg line failed because of light from both 254 and 194 nm lines leaking through the single 185 nm filter being used. Good measurements were made by either using two 185 nm filters (Acton 185-HR-1D-MTD) or one filter and a PMT with a Cs-I photocathode. A Cs-I
photocathode attenuates light at λ > 190 nm, and was used in penray measurements done by Cantrell et al. and Creasey et al on N2O absorption.6,7 Figure 5-3 shows the
absorbance plots for N2Oat 185 nm with the bialkali and Cs-I PMTs. The curvature was obvious in the bialkali case.
Figure 5-3. Curvature in the absorbance plot for N2O was seen when using the PMT with the bialkali photocathode, but not when using the Cs-I photocathode.
Static cell and Cary spectrometer measurements of the absorption cross sections for N2O and CF3Br were made to verify the values from the literature. The static cell measurements used the 208 cm path length that was fixed by the window – window distance of the kinetics cell. Each gas was allowed to fill the cell, and the total
concentration was determined from the pressure in the cell by using the ideal gas law. For the Cary measurements a 10.5 cm cell with NaCl windows was attached to the main kinetics cell to fill, and then was taken to the Cary. For each concentration 15 s averages were recorded 10 times and then those 10 recordings were averaged. The spectral bandwidth was 0.5 nm. Some clear nonlinearity could be observed in the Cary
absorbances, but these could be fit with equation (5.2) to give good cross section values.
y = 2.61E-17x + 2.19E-02 R² = 1.00E+00
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
0.00E+00 1.00E+17 2.00E+17
Absorbance
[N2O] molecules cm-3
N2O with bialkali PMT N2O with Cs-I PMT
(5.2) E is the error term associated with the nonlinearity and was usually around 0.05. All of the measured cross sections and their literature values are shown in Table 5-1. Good agreement was seen between the CF3Br and N2O cross sections measured and their literature values, verifying the overall absorption measurements.
5.2.2 [CH3OH] measurements
Using the procedure for the 185 nm absorption measurements established with N2O and CF3Br, measurements of [CH3OH] were made under normal flowing conditions for the kinetics flow cell. Concentrations of CH3OH measured by absorption agreed with the concentrations calculated from the flows when the total pressure in the CH3OH bubbler was kept above 300 Torr. At pressures < 300 Torr, [CH3OH] measured was larger than the calculated value. The 300 Torr mark was true for a variety of temperatures as can be seen in Figure 5-4. At 222 K, the lowest temperature on the plot, the absorption measurements became nonlinear at 1 x 1016 molecules cm-3. This was also true at 210 K at ~ 1 x 1015 molecules cm-3. Condensation or dimer formation of CH3OH was probably occurring, making these conditions approximate limits of temperature and CH3OH for this experiment.
Figure 5-4. [CH3OH] measured by the Hg lamp agrees with the concentration determined by the flows when the bubbler pressure is above 300 Torr.
The measured values were still slightly larger than the predicted ones. This data was analyzed with a path length of 138 cm, however most of the data fit even better using a path length between 145 and 155 cm. This is in agreement with the path length data discussed next, and is consistent with the current understanding of the flow system.